Method of growing rod-shaped dislocation-free monocrystals, particularly of silicon, by crucible-free floating zone melting

ABSTRACT

Method of growing rod-shaped monocrystals by floating zone melting includes adjusting the orientation of the seed crystal in its holder so that a main crystal axis thereof, which extends substantially in the longitudinal direction thereof, is inclined at an angle between 0.5* and 5* to the direction in which the rotary axis of the seed holder extends, and, at the start of the relative movement between the induction heating coil and the rod for passing a molten zone axially through the rod, the rod holder and the crystal holder are moved relatively away from one another so as to increase the spacing therebetween and form a bottleneckshaped constriction at the end of the rod to which the seed crystal is fused.

United States Patent Longo et al.

[151 3,655,345 [451 Apr. 11, 1972 Keller, Pretzfeld; Carl-Heinz Vogel, Belecke-Mohne, all of Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich, Germany [22] Filed: Apr. 8, 1968 [21] Appl. No.: 711,641

2,988,433 6/1961 Hanson ..23/30l 3,194,691 7/1965 Dikhoff ..23/30l 3,397,042 8/1968 Hunt ..23/273 FOREIGN PATENTS OR APPLICATIONS 848,382 9/1960 Great Britain ..23/301 Primary Examiner-Norman Yudkofi Assistant Examiner-R. T. Foster Attorney-Curt M. Avery, Arthur E. Wilfond, Herbert L. Lerner and Daniel J. Tick [57] ABSTRACT Method of growing rod-shaped monocrystals by floating zone melting includes adjusting the orientation of the seed crystal in its holder so that a main crystal axis thereof, which extends substantially in the longitudinal direction thereof, is inclined at an angle between 0.5 and 5 to the direction in which the rotary axis of the seed holder extends, and, at the start of the relative movement between the induction heating coil and the rod for passing a molten zone axially through the rod, the rod holder and the crystal holder are moved relatively away from one another so as to increase the spacing therebetween and form a bottleneck-shaped constriction at the end of the rod to which the seed crystal is fused.

3 Claims, 4 Drawing Figures METHOD OF GROWING ROD-SHAPED DISLOCATION- FREE MONOCRYSTALS, PARTICULARLY OF SILICON, BY CRUCIBLE-FREE FLOATING ZONE MEL'I'ING Our invention relates to method of growing rod-shaped, dislocation-free monocrystals of given material, particularly of silicon, by crucible-free floating zone melting a polycrystalline rod of the material. In such method, the rod is supported substantially vertically at an end thereof and an induction heating coil adjacent or surrounding the rod is movable relative to the rod in the longitudinal direction thereof. A rod-shaped monocrystalline seed crystal of the same material as the polycrystalline rod is inserted with accurately controllable orientation in a crystal holder that is rotatable about a substantially vertical axis, and the seed crystal is fused to the other end of the polycrystalline rod.

It has already been known to employ a method of adjusting a monocrystalline seed crystal during a process of pulling monocrystals, particularly monocrystalline semiconductor rods, whereby a narrow light band is projected onto a polished and oriented etched surface disposed substantially perpendicularly to the axis of the rod-shaped seed crystal. The orientation of the seed crystal is adjusted by means of a mechanical adjusting device so that the reflected light beam does not travel during rotation of the seed crystal about the rotary axis of the seed crystal holder. This method is employed for eliminating the normally occurring deviation of the [111]- direction of the seed crystal from the rotary axis thereof, whereby the semiconductor rod has an accurate [l11]-orientation after the pulling operation. The orientation of the seed crystal aimed at can also be used for adjusting any other selected orientation of the crystal growth, as disclosed in German Published Application 1,061,745.

It has been found that for a desired disorientation of the seed crystal which is dependent upon the direction of growth of the growing monocrystal, rod-shaped monocrystals can be produced that are free of dislocations over the entire length thereof. Success has been achieved particularly in so carrying out this method that monocrystals having very large diameters, such as diameters of 30 mm. and more, for example, can be grown free of dislocations.

It is accordingly an object of our invention to provide an improved method of growing rod-shaped, dislocation-free monocrystals, particularly of silicon, by crucible-free floating zone melting operations.

With the foregoing and other objects in view, we provide such a method wherein the seed crystal is adjusted so that a main axis of the crystal extending at least approximately in the longitudinal direction thereof in inclined at an angle between 0.5 and relative to the direction in which the rotary axis of the holder thereof extends, and, at the beginning of the longitudinal movement of the melting zone from the seed crystal through the rod-shaped semiconductor material, a bottleneckshaped constriction is formed by increasing the spacing of the rod holder from the holder of the seed crystal.

The disposition of a bottleneck-shaped constriction in the vicinity of the location at which the seed crystal is fused to the rod-shaped semiconductor material has been known, for example, from the German Published Application 1,079,593. This method step is effective in connection with the desired disorientation if the rod is to be pulled without dislocations therein. It has been found to be particularly advantageous to have an inclination angle of about 2 between one of the main crystal axes and a line extending in the direction of the rotary axis of the seed crystal holder.

The method of our invention can be easily supervised and controlled by an optical adjusting device and can be reproductively performed. A (111)-plane or a (111)-cleavage plane serves thereby as crystallographic reference value. If a seed crystal, namely, having a longitudinal axis which coincides substantially with the [l1l]-axis is clamped in a holder so that the longitudinal axis of the crystal shifts somewhat relative to the extension line of the rotary axis of the holder thereof, a light reflection with three bands emanating from a center point and offset about 120 relative to one another is produced when the end surface of the seed crystal is irradiated with a light beam extending in the direction of the rotary axis extension. In this regard, it is of particular significance that the light reflection from the outlet point or point of apparent origin of the light beam is so displaced that the light beam outlet point coincides with the extension of one of the three light bands beyond the center of the light reflection or the connecting line between the outlet point of the light beam and the center of the light reflection forms an angle of at most 15 with the aforementioned extension.

If the monocrystal grows in an upward direction from below, the light reflection can also be ofl'set from the outlet point of the light beam so that a band of the light reflection forms an angle between 15 and 45 and preferably 30 between the outlet point of the light beam and the center of the light reflection. This angle is formed when the original band lying on the connecting line between the exit or point of passage of the light beam and the center of the light reflection is rotated in the same rotary direction as the seed crystal holder about the center of the light reflection.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method of growing rod-shaped dislocation-free monocrystals, particularly of silicon, by crucible-free floating zone melting, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The method of the invention, however, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawing, in which:

FIG. 1 is a schematic perspective view of an optical adjusting device for a seed crystal employed in the growing of a dislocation-free monocrystalline rod by passing a melting zone in a downward direction through a polycrystalline rod;

FIG. 2 is a plan view of the collecting screen shown at the bottom of FIG. 1 showing the disposition of the light reflection for the desired disorientation of the seed crystal of FIG. 1 wherein the melting zone is being passed in a downward direction through a substantially vertically disposed polycrystalline rod;

FIG. 3 is the same view as FIG. 2, showing the disposition of the light reflection when the melting zone is being passed in an upward direction through the rod; and

FIG. 4 is a perspective view of a portion of a dislocation-free monocrystalline semiconductor rod having a bottleneckshaped constriction and showing a melting zone being passed downwardly through the rod.

Referring now to the drawing and first, particularly to FIG. 1 thereof, there is shown a lapped monocrystalline seed crystal 1, such as of a semiconductor material like silicon, for example, that has been etched in caustic potash or potassium hydroxide. The seed crystal 1 is clamped in an orientatable holder 2 which is securely mounted on an upper drive shaft 3 which is rotatable and axially displaceable. On a lower rotatable and axially displaceable drive shaft 4 there is mounted an illuminating device 5 surmounted by a diaphragm 6 which serves as a collecting screen for a light beam 7 that is reflected from the free end surface of the seed crystal 1. A light beam 9 emanating from the illuminating device 5 passes through the opening 8 of the diaphragm 6 and is reflected as the light beam 7 from the etched (111)-planes at the end face of the seed crystal 1. A light reflection 10 formed of three bands extending from a common point and offset 120 from one another is thereby produced. The position of these bands 10a, 10b and determines the position of the grown seams 11a, llb and 1 1c of the growing monocrystal 12 (note FIG. 4). The position of the [111 ]-direction of the seed crystal 1 is outlined by the beam 13 which respectively forms an angle 8 of about 2 with the light beams 7 and 9.

In FIG. 2 there is shown the position of the light reflection from the free end face of the seed crystal 1 onto the collecting screen 6 for a desired disorientation of the seed crystal 1 of about 2. The light reflection shown in FIG. 2 is produced when a dislocation-free monocrystalline semiconductor rod is being grown by passing a melting zone through a substantially vertically disposed polycrystalline rod of the semiconductor material from above in a downward direction along the rod. The light reflection is offset from the light beam 9 passing through the opening 8 of the collecting screen 6 and extends in a direction coinciding with that of one of the light bands, such as the light band 10b, for example. The deviation of the light reflection from this direction should be at most This limited range is indicated by the dotted lines in FIG. 2. The disorientation of the seed crystal 1 is thereby adjusted advantageously so that the ratio of the spacing b of the light reflection 10 from the outlet opening 8 for the light beam 9 to the spacing a (FIG. 1) between the free end face of the seed crystal 1 and the collecting screen 6 is For such a disorientation, the [UH-direction of the seed crystal 1 points to a band 10a of the light reflection 10 and is inclined less than an angle of 2 to the rotary axis of the drive shaft 3 of the seed crystal holder 2. Therewith, the [1 l1]-axis of the seed crystal 1 extends in the direction of one of the growth seams of the monocrystalline semiconductor rod 12 that is being grown. At this location, for the dislocation-free monocrystalline rod, a bulging 14 of the cross section of the rod (note FIG. 4) is formed, extending somewhat in the form of a rib along the monocrystal 13. This cross-sectional bulge 14 is a sure sign that the recrystallizing rod portion is free of dislocations.

In FIG. 3, there is shown the location of the light reflection 10 for a corresponding pulling or crystal-growing operation as in FIG. 2, but where the monocrystal is being grown in an upward direction from below. The light reflection 10 is offset from the outlet point 8 for the light beam 9 so that a band of the light reflection forms an angle of between 15 and 45, preferably 30, with the connecting line between the outlet opening 8 for the light beam 9 and the center of the light reflection 10. This angle is produced because the band lying originally on the connecting line between the outlet point 8 of the light beam 9 and the center of the light reflection 10 is rotated in the same rotary direction as the seed crystal holder 3 about the center of the light reflection 10.

The aforedescribed method of disorienting the seed crystal 1 can be employed for the most varied zone melting operations, for example with the so-called needles-eye method wherein the supply rod and the dislocation-free monocrystalline semiconductor rod to be grown are thicker than the inner diameter of the melting coil (see, for example, copending applications Ser. Nos. 564,118 and 664,211 now US. Pat. No. 3,414,388, both of W. Keller, filed respectively on July 11, 1966 and Aug. 29, 1967), or for conventional concentric as well as the more recently employed eccentric zone melting operations. It is especially advantageous if a bottleneckshaped constriction 15 is provided on the supply rod at the location at which the supply rod is fused to the seed crystal 1, when carrying out the aforementioned zone melting methods.

it is moreover important that the transition from the bottleneck-shaped constriction 15 to the aforedescribed nominal cross section of the growing monocrystal take place gradually. The speed of axial displacement of the holders of the rod and of the seed crystal relative to the induction heating coil 16, and the rotary speed of the holders must be maintained very constant, or variations must be carried out so slowly that the variation rate does not exceed 15 percent per second. Furthermore, it is important for the success of the method of our invention, that the high frequency current of the induction heating coil 16 be smoothed out to such an extent that it has at most a 1 percent ripple and has no modulations about 10 1-12. In addition, the effective value of the high frequency current must be regulated only so that it varies by at most a speed of 5 percent per second after the growing monocrystal 12 has attained its nominal cross section subsequent to the transition thereto from the bottleneck-shaped constriction 15.

We claim:

1. In a method of growing a rod-shaped dislocation-free monocrystal of given material by crucible-free floating zone melting a polycrystalline rod consisting of the same material and supported by a rod holder in a substantially vertical position, wherein the rod is fused at one end thereof to a rodshaped seed crystal of the same material that is inserted with accurately controllable orientation in a holder rotatable about a substantially vertical axis, and the rod and an induction heating coil located adjacent the rod are movable relative to one another in the axial direction of the rod so as to pass a molten zone formed at the fused end of the rod in a direction axially through the rod, the improvement which comprises adjusting the orientation of the seed crystal in the holder thereof so that a. main axis of the seed crystal extending substantially in the longitudinal direction thereof is inclined at an angle between 0.5" and 5 to the direction wherein the rotary axis of the seed crystal holder extends and, at the start of the relative movement between the induction heating coil and the red, the rod holder and the seed crystal holder are relatively movable away from one another substantially in the axial direction of the rod so as to increase the distance therebetween and form a bottleneck-shaped constriction at the fused end of the rod, the [lllj-axis of the seed crystal being oriented so that a light beam passing substantially perpendicularly through an outlet location of a light-collecting screen and along an extension of the rotary axis of the seed crystal holder and reflected from an etched end surface of the seed crystal, casts on the light-collecting screen a light reflection that is offset from the outlet location and is formed of three bands radiating from a center point and oflset from one another, the light reflection being so offset from the outlet location for the light beam that a connecting line between the light beam outlet location and the center of the light reflection forms an angle of at most 15 with an extension of one of the three bands beyond the center point of the light reflection.

2. In a method of growing a rod-shaped dislocation-free monocrystal of given material by crucible-free floating zone melting a polycrystalline rod consisting of the same material and supported by a rod holder in a substantially vertical position, wherein the rod is fused at one end thereof to a rodshaped seed crystal of the same material that is inserted with accurately controllable orientation in a holder rotatable about a substantially vertical axis, and the rod and an induction heating coil located adjacent the rod are movable relative to one another in the axial direction of the rod so as to pass a molten zone formed at the fused end of the rod in a direction axially through the rod, the improvement which comprises adjusting the orientation of the seed crystal in the holder thereof so that a main axis of the seed crystal extending substantially in the longitudinal direction thereof is inclined at an angle between 0.5 and 5 to the direction wherein the rotary axis of the seed crystal holder extends and, at the start of the relative movement between the induction heating coil and the rod, the rod holder and the seed crystal holder are relatively movable away from one another substantially in the axial direction of the rod so as to increase the distance therebetween and form a bottleneck-shaped constriction at the fused end of the rod, the [l11]-axis of the seed crystal being oriented so that a light beam passing substantially perpendicularly through an outlet location of a light-collecting screen and along an extension of the rotary axis of the seed crystal holder and reflected from an etched end surface of the seed crystal, casts on the light-collecting screen a light reflection that is offset from the outlet location and is formed of three bands radiating from a center point and offset 120 from one another, the growth of the rodshaped monocrystal being in a direction upwardly from below and the light reflection being ofiset from the outlet location for the light beam so that one of the three bands of the light reflection forms an angle between 15 and 45 with a connecting line extending between the outlet location for the light beam and the center point of the light reflection taken relative to the rotary direction of the seed crystal holder.

3. Method according to claim 2 wherein the angle between the one band of the light reflection and the connecting line is 5 30.

April 11, 1972 PO105 O m G O V Z N I d m L D R A C 9. 3 N a 0 M D mm 5 R s E G 3 B N !EA 5 G 5 S F 6 N 3 M w \IJ S O N I 0 t t R n e e t V a n Di I is certified hat said Letters that error appears in the above-identified patent P are hereby corrected as atent and shown below:

In the heading to the printed specification, line 11 March 8, 1968-- April 8, 1968" should read -F'iled:

' 5Filed:

(SEAL) Attest: I

EDWARD MEIETCHERJR. TEGTME EER Attesting Officer Acting Commissioner of Patents RENE D. 

2. In a method of growing a rod-shaped dislocation-free monocrystal of given material by crucible-free floating zone melting a polycrystalline rod consisting of the same material and supported by a rod holder in a substantially vertical position, wherein the rod is fused at one end thereof to a rod-shaped seed crystal of the same material that is inserted with accurately controllable orientation in a holder rotatable about a substantially vertical axis, and the rod and an induction heating coil located adjacent the rod are movable relative to one another in the axial direction of the rod so as to pass a molten zone formed at the fused end of the rod in a direction axially through the rod, the improvement which comprises adjusting the orientation of the seed crystal in the holder thereof so that a main axis of the seed crystal extending substantially in the longitudinal direction thereof is inclined at an angle between 0.5* and 5* to the direction wherein the rotary axis of the seed crystal holder extends and, at the start of the relative movement between the induction heating coil and the rod, the rod holder and the seed crystal holder are relatively movable away from one another substantially in the axial direction of the rod so as to increase the distance therebetween and form a bottleneck-shaped constriction at the fused end of the rod, the (111)-axis of the seed crystal being oriented so that a light beam passing substantially perpendicularly through an outlet location of a light-collecting screen and along an extension of the rotary axis of the seed crystal holder and reflected from an etched end surface of the seed crystal, casts on the light-collecting screen a light reflection that is offset from the outlet location and is formed of three bands radiating from a center point and offset 120* from one another, the growth of the rod-shaped monocrystal being in a direction upwardly from below and the light reflection being offset from the outlet location for the light beam so that one of the three bands of the light reflection forms an angle between 15* and 45* with a connecting line extending between the outlet location for the light beam and the center point of the light reflection taken relative to the rotary direction of the seed crystal holder.
 3. Method according to claim 2 wherein the angle between the one band of the light reflection and the connecting line is 30*. 